A superconducting magnet coil system of this kind is known from EP 138 270. The cited publication discloses a magnet coil system which is composed of superconducting coils and which comprises, notably for active self-shielding, two coaxially arranged coil systems which are to be activated with opposite polarity.
In such magnet systems, actively shielded or not, internal magnetic field variations can be induced due to magnetic field variations occurring outside the magnet. Magnetic field disturbances within the magnet are liable to disturb the imaging by means of magnetic resonance signals.
The invention is inter alia based on the recognition of the fact that superconducting MR magnets are customarily used in the so-called persistent mode where the magnet is short-circuited in itself and, if the magnet coil circuit is suitably superconducting, current and field will remain very stable over a prolonged period of time. When a magnet thus short-circuited is exposed to a varying, externally generated magnetic field, for example originating from a moving magnetized object or resulting from a varying electric current, the total magnetic flux enclosed by the magnet will remain constant. When the external field causes a magnetic flux in the magnet, the current in the magnet will change so that the variation of the flux within the magnet will be equal but opposed to the external magnetic flux. Within the magnet a current variation causes a field variation whose extent is co-determined by a coil constant of the magnet. This field variation is generally opposed to the external field variation.
It is to be noted that U.S. Pat. No. 4,974,113 discloses a coil system which aims to reduce the effect of external field variations on the measurement field homogeneity, but this solution leads to a comparatively long coil system.